Wavelength conversion technology that converts wavelengths of signal light is used, for example, to effectively design and operate photonic networks. Photonic networks utilize widely used wavelength division multiplexing and use optical fiber transmission lines as signal light transmission lines. The wavelength conversion technology desirably features an independent property from modulation formats so as to operate for signals of a variety of modulation formats independently of signal light modulation formats. It is also desirable that the wavelength conversion technology feature a wavelength conversion property that enables conversion of a wavelength into a desired wavelength and ultra high-speed response characteristics for handling high-speed signals.
In general, wavelength conversion of signal light is achieved by converting signal light into an electric signal with an optical receiver and modulating output light of a laser diode (LD), which outputs light at different wavelengths, with the converted electric signal. This method enables integration of electronic circuits, and may convert a wavelength into a desired wavelength if the LD that outputs light at the desirable wavelength is provided. However, with the wavelength conversion method of signal light using electronic circuitry, since the electronic circuitry depends on signal light modulation formats, it is difficult for a single electric circuit to deal with signals of various modulations. There is also a problem with high-speed response characteristics because of limitation on the operating speed of the electronic circuitry.
With the signal light wavelength conversion method that keeps signal light in the form of light without using electronic circuitry, the limitation on the operating speed of electronic circuitry is avoidable. Thus, the ultra high-speed response characteristics are achieved, and the dependency on modulation formats may be eliminated because of preserving the optical electric field in this method. In addition, a process of converting a signal light wavelength that keeps signal light in the form of light without using electronic circuitry may consume less electricity than a process of converting signal light wavelengths using electronic circuitry. For these reasons, there are growing expectations for a practically usable signal light wavelength conversion method that keeps signal light in the form of light without using electronic circuitry. The method has been widely researched and developed, and a variety of techniques has been proposed thus far.
FIG. 1 illustrates a wavelength conversion method using a four-wave mixing (FWM) effect that is produced in an optical nonlinear medium. The wavelength conversion method using the FWM effect is one of conversion methods that keep signal light in the form of light.
When pump light enters the optical nonlinear medium together with signal light, a copy of the signal light, referred to as idler light, is generated at a newly generated different wavelength λout by the nonlinear effect under a condition described later. The wavelength of the signal light is converted by extracting the idler light with an optical filter. Since the idler light generated is phase conjugate light of the signal light, the optical electric field intensity and the phase are preserved through the FWM effect. For this reason, a wavelength conversion method using the FWM effect is able to convert a wavelength of signal light of any modulation format without losing information. In addition, since the FWM effect is a nonlinear effect that responds in the femtosecond order, an ultra high-speed response is achievable. However, in order to efficiently produce the FWM effect, there is a condition that the wavelength of the pump light (λp) matches the zero dispersion wavelength inherent to the nonlinear medium. Under this condition, the wavelength of the newly generated light (λout) satisfies the following equation.
                                          1                          λ              out                                =                                    2                              λ                p                                      -                          1                              λ                                  i                  ⁢                                                                          ⁢                  n                                                                    ,                            (        1        )            
where λout, λp, and λin represent the wavelength of the idler light, pump light, and the input signal light, respectively.
Since the wavelength λp of the pump light is fixed, it is impossible to convert the wavelength λin of the input signal light into a desired wavelength of the output signal light with the wavelength conversion method illustrated in FIG. 1, as seen in the above equation (1).
FIG. 2 illustrates a wavelength conversion method using a self-frequency shift effect. The wavelength conversion method using the self-frequency shift effect is one of conversion methods that keep signal light in the form of light. The self-frequency shift effect is one of optical nonlinear effects.
The wavelength conversion method using the self-frequency shift effect utilizes the following phenomenon in which, when the signal light enters an optical nonlinear medium, the signal light itself functions as the pump light and the wavelength (frequency) of the signal light is shifted to a longer wavelength (lower frequency) by the Raman effect. A wavelength shift amount depends on optical power of the signal light input to the nonlinear medium. Thus, as illustrated in FIG. 2, the wavelength shift amount is adjustable by adjusting the power of the signal light input to the nonlinear medium with an optical amplifier, an optical attenuator, or the like.
In addition, since the wavelength conversion using the self-frequency shift effect utilizes a gain produced by the Raman effect, the optical electric field intensity and the phase are preserved. Since the Raman effect features a very high-speed response, this wavelength conversion is operable at a speed far higher than that of the operation of electronic circuitry. However, since the gain derived from the Raman effect is produced only on the long wavelength side of the signal light, the wavelength conversion using the self-frequency shift effect is unable to convert a wavelength into a wavelength on the short wavelength side.
Technology of four wave mixing is disclosed, for example, in Japanese Laid-open Patent Publication No. 2004-163558, and in “Nonlinear Fiber Optics”; Fourth ed., Govind P. Agrawal, Academic Press, 2007. A wavelength conversion technology using the self-frequency shift effect is disclosed, for example, in “Analysis of Widely Wavelength Tunable Femtosecond Soliton Pulse Generation Using Optical Fibers,” Nishizawa Norihiko, Okamura Ryuji and Goto Toshio, Japanese Journal of Applied Physics, Vol. 38, pp. 4768-4771, Part 1, No. 8, August, 1999.